This invention relates to differential buffers, and more particularly to self-biasing of differential buffers.
Differential buffers are a widely used building block in electronic systems. Input signals can be fully differential, with a true and a second, complement line that are driven to opposite states. Many kinds of low-voltage differential signaling (LVDS) have been developed and are available.
Differential buffers can also be used with single-ended input signals. The single input signal is compared to a reference voltage by the differential buffer. The reference voltage can be applied to the inverting (negative) input of the differential buffer while the input signal is applied to the non-inverting (positive) input.
FIG. 1 shows a differential buffer comparing an input signal to a reference voltage. Differential buffer 10 receives an input signal DIN on the non-inverting (+) input, and a reference voltage VREF on its inverting (−) input. Output Q is driven high when the input voltage of DIN is above VREF; otherwise output Q is driven low by differential buffer 10.
The reference voltage VREF can be set to the logic threshold level of input signal DIN. This switching threshold is often set to be half of the power-supply voltage, or VDD/2. Reset signal RST can be activated to reset the output.
Internal bias voltages are often used in complementary metal-oxide-semiconductor (CMOS) amplifiers and buffers. A voltage divider between power and ground can generate one or more bias voltages that are applied to gates of current source transistors in the differential amplifier. The voltage divider often is a series connection of transistors, and may include one or more resistors in series with the transistors.
The voltage divider generating the bias voltages has the disadvantage of no feedback to compensate for temperature, power-supply, and process variations.
What is differential buffer that generates its own bias voltages without an input from a voltage divider between power and ground. A self-biased differential buffer is desirable.